Cyclophosphamide is the generic name for 2-[bis(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorine-2-oxide monohydrate, a widely used antineoplastic drug chemically related to the nitrogen mustards. The nitrogen mustards are known as alkylating agents. These alkylating agents undergo strongly electrophilic reactions with such biologically important molecules as DNA. By alkylating DNA, these agents interfere with replication which ultimately interferes with cell viability. When exposed to these alkylating agents at low doses, the cell is capable of remaining viable by relying on its DNA repair enzymes which remove these alkylators prior to replication, thus allowing replication to proceed normally. The efficiency of these repair enzymes will be related to the degree and type of alkylation which must be repaired. For example, alkylation of a single strand of DNA may often be repaired with relative ease allowing normal cell division. However, damage to DNA caused by interstrand cross linkers (bifunctional agents such as cyclophosphamide) are more difficult to repair and involve more complex mechanisms. Thus, with increasing doses there is more extensive cross linking resulting in DNA breakdown with concomitant cell death.
These agents are cell cycle nonspecific being capable of combining with cells at any phase of their cycle. The therapeutic efficacy of these compounds arises from their interference with replication in cells which are dividing faster than their DNA repair enzymes can de-alkylate. It is through this destructive interference with replication in these rapidly dividing cells which these alkylating agents exert their cytotoxicity.
Cancer cells are notorious for being just such rapidly proliferating cells. This affords the pharmacologist a target at which chemotherapeutic agents may be aimed. Cancerous tissue growth outpaces the corrective effect of the DNA repair enzymes. These tissues then undergo extensive cell death due to breakdowns in replication and tissue growth falls off.
Cyclophosphamide was one example of a group of novel cyclic phosphoric acid ester amides which were disclosed and claimed in U.S. Pat. No. 3,018,302 granted Jan. 23, 1962 to H. Arnold et al.
A related series of compounds bearing substituents on the oxazaphosphorine ring nitrogen was disclosed and claimed in U.S. Pat. No. 3,732,340 granted May 8, 1973 also to H. Arnold et al.
Early in its clinical application cyclophosphamide was available as the monohydrate in parenteral dosage formulations consisting of sterile packaged dry powder blend admixtures of the drug and sodium chloride. The premixes were dissolved in water prior to administration which could be oral as well as parenteral. The aqueous solution, however, necessitated prompt administration in that shelf-life was limited to several hours after preparation. Moreover, during processing and/or storage of the dry powder premix formulation, a glassiness and/or stickiness could be acquired by the premix composition giving an unattractive material with inferior solubility characteristics and decreased potency. This deterioration was more pronounced as storage time was extended or if the upper limit of the storage temperature range was exceeded.
This temperature susceptibility was problematic in that a common practice in the constitution of sterile solids involves heating the mixture to expedite the dissolution process. It has been shown, however, that warming vials of cyclophosphamide in order to facilitate dissolution, after adding an aqueous vehicle, could decrease the potency of the final injectable product. D. Brooke et al., American Journal of Hospital Pharmacy 32:44-45 (1975). Subsequently, these stability limitations and dissolution difficulties were recognized as substantial shortcomings which often resulted in clinical use of subpotent cyclophosphamide solutions.
As a result of this thermal and hydrolytic susceptibility, workers in the art turned to lyophilization. The technique known as lyophilization is often employed for injectable pharmaceuticals which exhibit poor stability in aqueous solution. This process involves freeze drying, whereby ice is sublimed from frozen solutions leaving only the solid, dried components of the original liquid. On Aug. 27, 1985 a patent issued to R. L. Alexander et al. which disclosed and claimed process and preparations for lyophilized cyclophosphamide.
Lyophilization has several advantages over the previous dry powder formulations. Lyophilization permits pharmaceuticals which are unstable in aqueous solution, yet relatively stable in the solid state to be processed and filled into dosage containers in solution, taking advantage of the relative ease of processing a liquid; dried without elevated temperatures, thereby eliminating adverse thermal effects; and then stored in the dry state in which there are relatively few stability problems.
Lyophilization has several accompanying disadvantages as well. The lyophilization process is costly, inefficient and dangerous.
Lyophilization requires sophisticated vacuum pumps, sterile, refrigerated chambers with meticulous thermal controls for cooling samples, condensors to trap the water vapor as it sublimes from the frozen solution, and thermocouple probes for monitoring product temperature. The apparatus itself, the energy and the technicians required to run it become quite expensive on an industrial scale and raise the cost of the product accordingly.
Lyophilization is inefficient. Lyophilization involves removing water from a frozen aqueous solution leaving a freeze-dried solid, shipping the freeze-dried solid to the customer, who, when necessary, reconstitutes the drug as an aqueous solution. Clearly, if the lyophilization process could be circumvented, time and cost would be reduced.
The lypholized product must be reconstituted. Reconstitution necessitates some degree of personnel exposure. This is particularly undesirable when the drug is a strongly cytotoxic antineoplastic agent. This hazardous personnel exposure is aggravated by aerosolization of the potent cytotoxic agent. As a lyophilizate the drug must be dissolved prior to removal for injection. This necessitates additional entry to the vial with a syringe to add the solubilizing liquid vehicle. With each accession of the vial small quantities of the drug become airborne and this is known as aerosolization. Such added exposure requires particular precautions such as rubber gloves and masks. Furthermore, reconstitution introduces potential for dilution errors. For these and other reasons producers and consumers alike prefer readily injectable liquid formulations of parenterally administered drugs.
Heretofore, it has been generally recognized that liquid formulations of cyclophosphamide would not be possible due to the inherent instability of cyclophosphamide in water. It was unexpectedly discovered, however, that stable solutions of cyclophosphamide for parenteral or oral administration are, in fact, possible. This occurs when cyclophosphamide is dissolved in a solution containing an organic solvent, such as a polyol, preferably propylene glycol, polyethylene glycol or glycerol, or combinations thereof. (See Tables 1 and 2.)